A packet radio system refers to a radio system employing packet switched technique known from the fixed networks. Packet switching is a method wherein a connection is set up between the users by transmitting data in packets that comprise address and control information. A plurality of connections can simultaneously use the same transmission link. Research has been carried out particularly on the use of the packet switched radio systems since the packet switching method is well suited for data transmission wherein data to be transmitted is generated in bursts. Consequently, the data transmission link does not have to be continually allocated, only for transmitting the packets. Cost and capacity can thus be considerably saved both when the network is being built and used. The present packet radio networks are particularly attractive to the further development of the global system for mobile communication GSM, known as a general packet radio service GPRS.
The present invention relates to a method and a telecommunication system by means of which a base station system in accordance with the GPRS system can control its load level in a controlled way such that the telecommunication system is not subjected to overload. In the GPRS system, load in the base station system of the telecommunication system is mainly directly or indirectly caused by channel allocation requests transmitted by the subscriber terminals to the mobile telephone network. It is previously known to prevent overload by a flow control procedure, for example, by means of which the other party can be restrained from transmitting more data than the system can receive and process. An example of how to restrict the subscriber terminal's capability to make channel allocation requests in this way is to clearly prohibit the terminal from trying to reserve radio resources dung a given period of time after the subscriber terminal has already transmitted a channel allocation request to the network. If, for example, all channels in a base station are already in use, the base station controller can transmit a “reject” type of message to the subscriber terminal, which has already transmitted a channel allocation request to the network. The “reject” message can, for example, indicate the time after which the subscriber terminal is allowed to retry channel reservation at the earliest.
A second previously known method for controlling load is an experimental study to find out how high channel allocation request load a base station can tolerate. The next step is to provide a filtering algorithm which starts filtering off the channel allocation requests when a given predetermined load limit is exceeded.
A third way to limit the channel allocation requests is to parametrize the base station system in a suitable manner. The base station system can, for example, change its system parameter information over the broadcast control channel BCCH and thus affect the subscriber terminals' capability to make channel allocation requests. “Random access channel RACH control parameter” system parameter information can affect, for example, whether channel allocation requests can be made to a cell at all or whether only channel allocation requests of certain access control classes are allowed. It is also feasible, for example, to prevent emergency calls from being made via a cell, and to affect the repetition frequency at which channel allocation requests are allowed to be made. Furthermore, it is also possible to affect the number of retries allowed in case the subscriber terminal does not receive a response to its channel allocation request from the mobile telephone network at its first attempt.
The systems described above suffer from the following problems. In the first method, the base station system may, in principle, receive simultaneously so many channel allocation requests from the subscriber terminals that the base station system does not have the time to react sufficiently quickly to the situation, but the system becomes overloaded. The method thus cannot restrict the effort to try channel reservation from subscriber terminals that have not even tried channel reservation.
Since the second method requires empirical study into an already operating system, a suitable filtering algorithm cannot be provided in advance. The suitable limit must therefore be found experimentally. The method can only restrict load generated by channel allocation requests, therefore load caused by other functions cannot be taken into account in the filtering algorithm. Mere filtering off a channel allocation request also quickly generates new channel allocation requests to the same base station since the subscriber terminal, failing to receive an appropriate response to its first request, will retry channel reservation. This further increases the number of channel allocation requests to be received.
Depending on the situation, the third method applies different principles when allocating radio resources to different types of subscribers, rather than trying to prevent the base station system from becoming physically loaded.